When light propagates through a given homogeneous medium,the velocities of

The figure shows plane waves refracted from air to water using Huygens's principle. $a, b, c, d, e$ are lengths on the diagram. The refractive index of water with respect to air is the ratio:

The figure shows a surface $XY$ separating two transparent media,medium-$1$ and medium-$2$. The lines $ab$ and $cd$ represent wavefronts of a light wave travelling in medium-$1$ and incident on $XY$. The lines $ef$ and $gh$ represent wavefronts of the light wave in medium-$2$ after refraction.
$1.$ Light travels as a
$(A)$ parallel beam in each medium
$(B)$ convergent beam in each medium
$(C)$ divergent beam in each medium
$(D)$ divergent beam in one medium and convergent beam in the other medium.
$2.$ The phases of the light wave at $c, d, e$ and $f$ are $\phi_{c}, \phi_{d}, \phi_{e}$ and $\phi_{f}$ respectively. It is given that $\phi_{c} \neq \phi_{f}$.
$(A)$ $\phi_{c}$ cannot be equal to $\phi_{d}$
$(B)$ $\phi_{a}$ can be equal to $\phi_{e}$
$(C)$ $(\phi_{d}-\phi_{c})$ is equal to $(\phi_{f}-\phi_{e})$
$(D)$ $(\phi_{d}-\phi_{c})$ is not equal to $(\phi_{f}-\phi_{e})$
$3.$ Speed of the light is
$(A)$ the same in medium-$1$ and medium-$2$
$(B)$ larger in medium-$1$ than in medium-$2$
$(C)$ larger in medium-$2$ than in medium-$1$
$(D)$ different at $b$ and $d$
Give the answer for questions $1, 2$ and $3$.

Rays diverging from a point source form a wave front that is:

According to Huygens principle,the phase difference between oscillations of any two particles on the same wave front is . . . . . . rad.

Explain the wavefront and its types.